In the cities of Navotas and Malabon in the densely populated area of northern Metropolitan Manila in the Philippines, flooding has become common.
So much so, in fact, that the country’s distinctive jeepney vehicles (elaborately decorated buses) are now made from stainless steel to prevent corrosion by seawater. The streets have been repeatedly elevated too, to the point that some are now higher than people’s doors.
“They keep on putting the roads higher and higher, but I don’t know how they can sustain that,” says Dr Mahar Lagmay, executive director of the University of the Philippines Resilience Institute.
The struggle to keep above water isn’t just a result of the sea levels going up; if anything, it has more to do with the ground level going down.
A study (currently under review) carried out by Lagmay and his team suggests that between 2014 and 2020 parts of Metro Manila sank by as much as 10.6cm (4.2in) a year. That’s 24 times the global average of sea-level rise, which is 0.44cm (0.17in) a year.
According to Lagmay, the region’s rapid descent first became apparent in the late 1990s, when scientists plotting data from tidal gauges found that the water levels they recorded couldn’t be explained by climate change alone.
“It was too much just for sea-level rise to be the culprit,” he says. In the years since, the situation has become untenable for coastal communities in certain parts of Manila Bay: flooding has left homes half underwater and rice farmers have turned to fishing.
Manila is far from the only highly urbanised area affected, though. The alarming rate at which it’s dropping isn’t unheard of, according to subsidence expert Dr Matt Wei from The University of Rhode Island, in the US, who has studied sinking cities on a global scale.
Places all around the world are falling victim to the ‘big sink’. Jakarta in Indonesia is perhaps the prime example, with historical rates of sinking once reported to have reached 25cm (9.8in) per year in some areas. Worryingly, notes Wei, the sinking isn’t always accounted for in the models used to predict flooding.
How big is the big sink?
Nowadays, sinking measurements no longer come from tidal gauges. Lagmay, Wei and other researchers can make far more accurate estimates using satellite data.
One approach is to track 3D movements of the ground at defined points, like stations within a Global Navigation Satellite Systems (GNSS) network.
Scientists can also get a detailed picture of vertical movement across an entire land surface with a technique called Interferometric Synthetic Aperture Radar (InSAR), which uses the reflections of radar signals to calculate downward movement over time.
NASA and the European Space Agency make InSAR data openly available, which benefits developing nations like the Philippines. “It’s just a matter of using the data that’s available to everybody,” says Lagmay.
“You can use it as a monitoring tool to identify the places in metropolitan cities where there’s ground subsidence and focus your attention on those places.”
In a 2022 paper, Wei and his team combined GNSS and InSAR data to study sinking in coastal cities globally. Of the 99 cities they looked at, exactly one-third were sinking and, in parts, at least five times faster than sea levels were rising.
Many with high sinking rates were in Asia, such as the densely populated Mumbai, in India, as well as Chittagong, in Bangladesh (both 2cm, or 0.8in, per year), and Karachi, in Pakistan (1cm, or 0.4in). Meanwhile, satellite data from a study published earlier this year suggests that around 67 million people across China live in urban areas that are sinking faster than 1cm (0.4in) per year.
Even the world’s richest and most famous cities aren’t immune to the big sink. Efforts by the city of Venice, in Italy, to stop its piazzas and gothic architecture from disappearing under the Adriatic have been well publicised.
Fewer may be aware, however, that New York City, in the US, lies barely above sea level and is also sinking, albeit only by 0.1 to 0.2cm (0.04 to 0.08in) each year.
What are the causes?
In the case of New York, the sinking is partly a consequence of post-glacial rebound, a natural process that causes the ground beneath a melting glacier to rise as it emerges from under the weight of the ice while nearby areas simultaneously subside.
Some 20,000 years ago, New York would have been near the edge of a glacier and when it disappeared, as Wei explains, “the region where it used to be started to go up, while the edge went down.”
As for Metro Manila, other natural processes may be at play. An active volcano is simmering barely 50km (31 miles) away and earthquake fault lines run north to south through the area. But while Lagmay acknowledges that cracking and subsidence can be seen in buildings along the fault lines, he believes the sinking to be more closely linked to human activity.
A potential culprit is the sheer weight of the buildings. The effect is difficult to measure, however, since it’s not always clear what type of foundations or ground those buildings are resting on.
Studies in Rotterdam in the Netherlands suggest that even high-rises over 120m (390ft) only cause sinking of a few millimetres, at most, each year. And while Wei’s team tried to model the contribution that all the buildings in New York make to subsidence, the results seemed to overestimate reality.
This is possibly because most of the subsidence effect that buildings have takes place as they initially ‘settle’, so won’t be seen in older structures like those in New York.
Instead, Lagmay contends that Manila’s sinking is mainly due to excessive pumping of groundwater, which drains and weakens the ground.
“When we look at the images where there’s the greatest amount of subsidence, it coincides with industrial and commercial complexes,” he says, adding that although laws exist to prevent unsustainable extraction, enforcing them is another matter.
Groundwater extraction has also been blamed for sinking cities in China and, according to Wei, it’s “the most common reason” behind the problem globally. In some cities, illegal wells are the issue, such as in Jakarta, where piped water has historically been in short supply.
Can the big sink be stopped?
Some cities have managed to slow the sinking by getting a handle on groundwater extraction: as well as Jakarta, Tokyo in Japan and Houston in Texas have seen varying levels of success by tightening water regulations. Between 2015 and 2020, Jakarta’s once-rapid rate of sinking slowed to 3cm (1.2in) a year.
In Manila, the water board has outlawed the digging of deep wells, while alternative water supplies, including a lake south of the city, are helping to reduce pumping from wells that are responsible for local subsidence.
But in areas that are already flooded – like the village of Sitio Pariahan north of Manila, where residents take boats to get to work or school – people are coming to accept that the only solution is relocation.
Yet, the situation isn’t hopeless. The ground is able to rebound to some extent when it absorbs rainwater, as Wei saw recently when studying subsidence in Taipei, Taiwan. “[The ground] is just going down and back like a spring,” he says, explaining that recent sinking here seems to be partially related to drought.
Whether Manila can bounce back will depend on the state of the city’s aquifers (the water-bearing rock underground), says Lagmay. The question is if they have collapsed beyond the point of no return. “There needs to be more studies,” he admits. “But if you take care of it, monitor it and don’t over extract, maybe it can come back to its original state.”
About our experts
Dr Matt Wei is an associate professor of oceanography at the University of Rhode Island. His interests lie in tectonic geodesy, fault mechanics and natural hazards.
Dr Mahar Lagmay is the executive director of the University of the Philippines Resilience Institute. He has been involved in a number of projects aiming to future-proof the Philippines, including around flooding and groundwater.
Read more: